In integrated circuits, voltage controlled oscillators (VCOs) are an integral part of phase-locked loops (PLLs) contained in such integrated circuits and are important in communication circuits. For example, a voltage controlled oscillator (VCO) is often used to generate local oscillator signals used by transmitter and receiver subsystems for frequency up conversion and down conversion, respectively. An inductor-capacitor (LC) tank PLL is used in clock data recovery circuits in order to achieve very good jitter performance.
An LC tank VCO is specifically designed to operate at a specific frequency or a specified range of frequencies. The frequency of oscillation for the LC tank VCO may deviate from a nominal design value due to various factors such as circuit component tolerances, i.e., on chip spiral inductor variation, varactor variation, integrated circuit process variations, and circuit component aging. The oscillation frequency may also change during normal operation due to various factors such as temperature and power supply variation. So in any LC tank VCO external coarse tuning is required in order to adjust a center frequency of the VCO to compensate for process, voltage and temperature (PVT) variations. This coarse tuning can be done manually, that is externally switching on/off coarse tuning bits to readjust the center frequency or can be done automatically on chip.
FIG. 1 illustrates a prior art automatic frequency correction phase-locked loop (PLL) circuit. The circuit includes an analog control circuit and a digital control circuit. The digital control circuit includes a high-side comparator and a low-side comparator which receive an analog control voltage, a state monitor circuit, and a counter and decoder circuit. At least one of the high-side comparator and the low-side comparator includes a threshold switching circuit which selectively gives a first threshold and a second threshold having different values. When the analog control voltage remains between the high-side threshold and the low-side threshold in a state in which the threshold switching circuit gives the first threshold, the threshold switching circuit switches the first threshold to the second threshold and expands the interval between the high-side threshold and the low-side threshold.
In this type of conventional architecture, when the digital tuning has been completed and the analog tuning takes over the control voltage can sit at a very low value or at a very high value to achieve the required center frequency. This can cause a lot of VCO gain variation depending on the digital code that has been set during the coarse lock. Secondly, after the digital coarse loop has done its operation and the analog fine loop takes over, if there are some variations in the frequency due to temperature and voltage only the analog fine loop has to correct them, the digital coarse loop does not correct them. The digital coarse loop only corrects for the process variation in the VCO component values.
There is a need for a novel architecture that automatically switches on the coarse tuning loop, whenever needed, to compensate for the PVT variations by constantly monitoring the control voltage.